Icon+Unicon/Intro

Introduction to Icon and Unicon

Under Construction Notes

This page is a work in progress. The objectives of this page are as follows:

• Provide a place to put detailed documentation or descriptions that can be referenced from task pages or category pages
• Provide something akin to a readers digest condensed version of the books and other references
• Some of the information may be more appropriately moved to Paradigm pages

The initial text of this page has been taken from Flow-control_structures#Icon_and_Unicon

Purpose

The purpose of this page is to provide a Rosetta Code users with a enough supporting detail about Icon and Unicon to facilitate understanding and appreciation of these languages. It would be expected that the level of detail would be significantly smaller than any of the online books and reference materials listed on the Icon and Unicon pages.

Some of the sections should be referenceable from tasks.

Icon and Unicon Differences

The Icon Programming Language was the successor of a series of non-numeric languages including Commit, SNOBOL, SNOBOL4, and SL-5. Icon provided several innovations and improvements over its predecessors including: integrating the the powerful pattern matching capabilities of SNOBOL4 into a procedural language, banishing a number of unfortunate foibles, retaining the flexibility of a typeless langauge, reining in some programming side effects, keeping platform independence. Icon was one of the first bytecode languages undergoing continuous improvement from it's inception in the late 1970s.

Over the years various improvements, extensions, and experimental variants were added including a platform independent graphics interface, IDOL (an object oriented pre-processor), MT Icon (a Multi-Threaded variant), Jcon (an implementation in Java), and others. And while the graphics interface was integrated into Icon, many of these variants were not.

The Unicon Programming Language integrated a number of these extensions into a single variant of Icon. Unicon includes object-oriented support, improved system interfaces, and messaging. Additionally, a number of syntactic improvements to the language and semantic extensions to some functions have been implemented. Because of this Unicon isn't completely a superset of Icon.

Variables, Data Types, and Structures

un-Declarations

Icon and Unicon do not require strict static typing of variables as does languages like Pascal, nor does it require type definitions to reserve space such as in languages such as C. In fact, variables may happily change type from one moment to the next. Knowing this you might expect that declarations are non-existent.

Declarations are optional and any undeclared variables are either (a) parameters to procedures or (b) local to procedures. This design decision ensured that Icon/Unicon are not susceptible to the kind of side-effects that the global nature of variables in [SNOBOL4] led to.

Still, declarations are desirable for clarity and needed for a number of special cases:

• local - variables local to a procedure
• static - permanent variables that transcend individual calls of procedures. While not visible outside of a procedure they are visible between different instances and across recursion of a procedure.
• global - variables with global visibility

Additionally, the following declarations apply to non-variables:

• record - used to define a structure with named fields
• procedure - used to define a procedure and its parameters
• invocable - used to control program linking to ensure procedures are included and available if they are called (e.g. through string invocation)
• class - used to define an object class (Unicon)

Program Flow

Text from Flow Control Task

Prelude about Goal-Directed Evaluation and Generators

Some of this should likely belong under Category:Programming_paradigm/Logic_Programming

A central feature of Icon and Unicon is what is known as Goal-Directed Evaluation, and the intimately related concept of Generators. Without trying to be a tutorial, the idea is that expressions can yield more than one result (Generators) and if a further part of the expression results in failure, the earlier Generators will be driven to yield more results. The effect is not unlike the backtracking found in Prolog or Regular Expressions, however the feature is built into the very core of the language. Prolog programmers will find it very familiar but of course with differences because Icon and Unicon do not use the functional language pattern matching technique of Prolog.

To cut a long story short, every expression and statement can fail, or generate one or more results that can be consumed or applied in further parts of the statement. There are also a few keyword statements which force the generation of all possible outcomes for a statement or expression.

Another way of looking at it is to understand that every expression can yield a result sequence that can be empty; any code using this expression may choose to ask for more results, gather them into a container or aggregate, or choose to use one value and then move on without asking for all possible results.

goto

Does not exist in the Icon or Unicon language.

next

Restarts the enclosing loop. The conditional on the loop is evaluated as normal.

break expr

Default value of expr is the null value &null. This operator breaks out of the enclosing loop, yielding the expression as the result of the loop. Normally loops yield a failure ie no result, so you can write code like this:

The expression given to break can be another break, which effectively lets you break out of two levels of loop. Finally, the expression given to break can be the next command; breaks out of two levels of loop and re-enters the top of the third-level enclosing loop.

return expr

Default value of expr is &null. Apart from the usual meaning of return, if the expr value fails, then the procedure actually fails too, ie does not yield a value. See description of fail keyword. If the expr is capable of yielding more than one result, only the first result is asked for and used.

fail

Causes the the enclosing procedure to terminate without returning value. This is different from returning void or a null value that many other languages do when the code does not return an actual value.

The value of x will not be replaced if ftn() issues the fail command. If ftn fails, then Goal-Directed Evaluation will also fail the assignment, therefore x is not assigned a new value. If the flow of control through a procedure falls off the end, the procedure implicitly fails.

suspend expr

Default value of expr is &null. Any procedure containing the suspend command will yield a value to the calling code. However the procedure remains in a state of suspended animation ready to be reactivated if the calling code demands another result due to Goal Directed Evaluation. Note that this capability is built directly into the runtime rather than being an artifically constructed behaviour provided by Python or C#'s use of the 'yield' keyword. Every and all expressions may suspend or be involved in a suspending expression without any effort. Behaviourally much closer to Prolog which also supports backtracking as a core part of the language. If the expr is capable of yielding more than one result, then supend (if driven) will progressively yield all of those values.

A procedure can contain several uses of suspend and it's quite reasonable for the procedure to execute many of them in any chosen order.

stop(expr)

Terminate program with prejudice.

error trapping

The keyword &error is normally zero, but if set to a positive value, this sets the number of fatal errors that are tolerated and converted to expression failure; the value of &error is decremented if this happens. Therefore the now-common TRY-CATCH behaviour can be written as:

Various idiomatic simplifications can be applied depending on your needs.

error throwing

Errors can be thrown using the function